1. Field of the Invention
The present invention relates to inkjet printheads and printing apparatuses using the same. More particularly, the present invention relates to an inkjet printhead substrate on which electrothermal transducers for generating heat energy necessary for ejecting ink and drive circuits for driving the electrothermal transducers are formed, a printhead, and a printing apparatus using the printhead.
2. Description of the Related Art
In general, electrothermal transducers (heaters) of a printhead mounted in an inkjet-based printing apparatus and drive circuits for driving the electrothermal transducers are formed on one and the same substrate using a semiconductor processing technique, as described in, for example, U.S. Pat. No. 6,290,334. One proposed example of such a printing apparatus has a printhead in which an ink supplying port is provided near the center of a substrate, and heaters are provided at positions facing each other with the ink supplying port provided therebetween.
FIG. 1 schematically illustrates circuit blocks and ink supplying ports of an inkjet printhead substrate (element substrate) 110 of this type.
Referring to FIG. 1, six ink supplying ports 111 are formed on the element substrate 110 formed of semiconductor. In order to simplify the drawing, a circuit block 115 including only one ink supplying port 111 at the left is illustrated, and circuit blocks 115 corresponding to the remaining five ink supplying slots 111 are only schematically illustrated. In the circuit block 115 corresponding to the ink supplying port 111 at the left, heaters 112 are arranged in an array at positions facing each other with the ink supplying port 111 provided therebetween. Drive circuits 113 for selecting and driving the corresponding heaters 112 are provided corresponding to the heaters 112. Pads 114 for applying power and signals to the heaters 112 and the drive circuits 113 are arranged at edges of the element substrate 110.
FIG. 2 schematically illustrates the circuit structure of one drive circuit 113 shown in FIG. 1 and the flow of a signal.
Data including image data applied to the pads 114 is connected to a shift register 204 and a decoder 205 included in an internal circuit via an input circuit 201. In this example shown in FIG. 2, the input data is applied as serial data, and the serial data is converted into parallel data by the shift register 204. The image data included in the converted parallel data is input via a latch (not shown) to a plurality of heater driving blocks 206 (eight heater driving blocks 206 are arranged in this example). The shift register 204 has the function of a block selecting circuit 203 for selecting the validity/invalidity of the heater driving blocks 206. Another portion of the converted parallel data is supplied to the decoder 205 disposed next to the shift register 204. The decoder 205 has the function of a time-division selecting circuit 202 for outputting a time-division selection signal for sequentially selecting heaters driven in the heater driving blocks 206.
FIG. 3 is a circuit diagram of the interior of one heater driving block.
A heater driving block 300 includes heater-driving metal-oxide semiconductor (MOS) transistors 305, level conversion circuits 304, and heater selecting circuits 305, which are arranged corresponding to heaters 306 arranged in an array. A heater power supply voltage (first power supply voltage) is applied from the outside to heater power supply lines 301. The heater-driving MOS transistors 305 perform the function of a switch for allowing or preventing the flow of current to the corresponding heaters 306. A block selection signal 302 and a time-division signal 303 are input to an AND gate serving as each of the heater selecting circuits 305. When these two signals become active, the output of the AND gate becomes active. The voltage amplitude of an output signal of the AND gate is converted by a corresponding one of the level conversion circuits 304 to a power supply voltage (second power supply voltage) that is higher than a drive voltage (third power supply voltage) at a level of a logic circuit including the input circuit and a corresponding one of the heater selecting circuits 305. The level-converted signal is applied to the gate of a corresponding one of the heater-driving MOS transistors 305. Current flows through the heater 306 connected to this heater-driving MOS transistor 305 in which the voltage has been applied to the gate thereof, thereby driving this corresponding heater 306.
The voltage amplitude of the output signal is converted to the higher second power supply voltage because of the following reason. By increasing the voltage applied to the gate of the heater-driving MOS transistor 305, the on-resistance of the heater-driving MOS transistor 305 is reduced. Accordingly, current can more efficiently flow through the heater 306.
If possible, the voltage value of the second power supply voltage should not exceed a circuit breakdown voltage and a gate breakdown voltage of the MOS, and the second power supply voltage should be set to as high a value as possible. Further, if possible, the voltage value of the second power supply voltage may be the same as the voltage value of the first power supply voltage, which is the voltage value of the heater power supply lines 301. In most cases, however, the power supply voltage applied to heaters is generally set to a relatively high value, such as 20 V or greater. In most cases, a complementary metal-oxide semiconductor (CMOS) inverter is generally processed to have a breakdown voltage of about 15 V. Since the gate breakdown voltage of a MOS depends on a gate oxide film, the gate breakdown voltage of the MOS must be sufficiently lower than the withstand voltage of the gate oxide film. It is therefore often difficult to match the optimal voltage of a voltage conversion circuit with a heater driving voltage. U.S. Pat. No. 6,971,735 describes an example in which a voltage input from the outside of a substrate is adjusted by making the thickness of a logic circuit on a printhead substrate thinner than the thickness of a heater driving section.
In this case, if a power supply line for supplying the second power supply voltage, which is different from the heater power supply voltage (first power supply voltage), is additionally provided, the cost of the overall system is increased.
In order to solve this problem, a power generation circuit for generating a desired second power supply voltage from a heater power supply voltage (first power supply voltage) is provided in the interior of a printhead substrate. An example of this type of circuit is described in Japanese Patent Laid-Open No. 11-129479. Furthermore, U.S. Pat. No. 6,712,437 describes an example of a circuit in which an input voltage VDD for a logic circuit is input to a heater-driving switching element. However, the description does not concern the relationship between a heater power supply voltage VH and a VDD circuit.
FIG. 9 illustrates an example of a power generation circuit.
The circuit shown in this example includes an nMOS transistor 803 and a resistor 804 constituting an nMOS source follower, and resistor dividers 801 and 802. The heater power supply voltage (first power supply voltage) is divided by the resistor dividers 801 and 802, and the divided voltage is applied to the gate of the nMOS transistor 803. An output of the source follower serves as the second power supply voltage. With the resistor dividers 801 and 802, the voltage applied to the gate of the nMOS transistor 803 can be set to a desired value. Accordingly, the second power supply voltage can be a voltage that is lower than the heater power supply voltage (first power supply voltage).
FIG. 5 is a circuit diagram of the internal structure of a level conversion circuit and its peripheral circuits. In FIG. 5, a heater-driving MOS transistor 505, a heater 506, a PMOS transistor 510, and an NMOS transistor 511 are shown.
A signal supplied from a heater selecting circuit is inverted by an inverter operating at the third power supply voltage to generate an inverted logic signal, and this generated signal is applied to the gate of an NMOS transistor and a PMOS transistor operating at the second power supply voltage. The transistors driven by the second power supply voltage need to be elements that can withstand the second power supply voltage.
As another circuit structure, a structure in which level conversion is performed immediately after an output of a shift register and a decoder is proposed.
FIG. 4 is a circuit block diagram of the structure in which level conversion is performed immediately after a shift register and a decoder.
Referring to FIG. 4, pads 401, a time-division selecting circuit 402, and a block selecting circuit 403 are shown. The point that is different from the foregoing circuit structure is that output signals of a shift register 404 and a decoder 405 are level-converted by level conversion circuits 411 and 412, respectively. Circuits driven by the third power supply voltage, which has the same voltage amplitude and the same potential as an input signal, are those enclosed by line 415. Circuit blocks driven by the second power supply voltage higher than the level-converted first power supply voltage are those enclosed by line 416. These circuit blocks enclosed by line 416 include heater driving blocks 406.
With this structure, it becomes unnecessary to dispose a level conversion circuit for each heater selecting circuit. Accordingly, the density of circuits near heaters become increased, and the layout area becomes reduced.
As has been described above, in circuits on an inkjet printhead semiconductor substrate, the third power supply voltage which has the voltage amplitude of an input signal and which activates logic circuit blocks is used. Further, the higher second power supply voltage applied to the gate of a MOS transistor, which is a switching element for controlling heater current, is used. The circuits are controlled and driven by these two power supply voltages. Further, an output signal of a drive circuit for supplying the third power supply voltage is converted by a level conversion circuit into a signal with the signal amplitude of the second power supply voltage.
The first and third power supply voltages are supplied from a printer body to the printhead semiconductor substrate. In most cases, the second power supply voltage is generated by converting the first power supply voltage via a power supply voltage generation circuit provided in the substrate to a voltage lower than the first power supply voltage.
The sequence of supplying these voltages to the printhead semiconductor substrate is such that, after the third power supply voltage is applied, the heater power supply voltage (first power supply voltage) is applied. This is because, if the heater power supply voltage (first power supply voltage) is applied in a state where no third power supply voltage is applied, the head may operate unexpectedly.
That is, in a state where the first power supply voltage is applied, the second power supply voltage is also applied inside the substrate. Therefore, the heater driving circuit including the level conversion circuit is enabled. In contrast, an input signal of the level conversion circuit is output from a circuit that operates based on the third power supply voltage. However, in a state where no third power supply voltage is applied, the logic thereof becomes indefinite. In this state, the logic of an output of the level conversion circuit becomes indefinite, which may result in the logic where an unexpected heater is turned on.
In order to avoid this indefinite logic state, it is necessary to supply the third power supply voltage and then the first power supply voltage, which is followed by generation of the second power supply voltage in the substrate. In order to apply the voltages in this sequence, special measures must be taken by the printer body, resulting in an increase in the cost.